1. Field of the Invention
The present invention relates to a liquid crystal display device having an optical retardation plate attached to a liquid crystal panel in which liquid crystal molecules are aligned substantially vertically to attain a black display.
2. Description of the Related Art
Liquid crystal display devices are applied to various fields of OA equipment, information terminals, watches, televisions and the like because of their features of lightness, thinness, and low power consumption. Particularly, an active matrix liquid crystal display device is a liquid crystal display device which has an excellent response characteristic obtained by using thin film transistors (TFTs) for switching pixels. Thus, the active matrix liquid crystal display device is used as a monitor display for portable TVs or computers to display a large quantity of image information.
In recent years, with an increase in the quantity of information, there has been a demand for enhancement of resolution and display speed of liquid crystal display devices. High resolution is achievable by miniaturizing the TFT array structure and increasing the number of pixels.
On the other hand, consideration has been given to increasing the display speed by replacing a conventional display mode with another one. The other display mode may be selected from optically compensated birefringence (OCB), vertically aligned nematic (VAN), hybrid aligned nematic (HAN), and π-alignment modes using a nematic liquid crystal, and surface-stabilized ferroelectric liquid crystal (SSFLC) and Anti-Ferroelectric Liquid Crystal (AFLC) modes using a nematic liquid crystal, for example.
Of these display modes, the VAN mode in particular has advantageous features that can obtain a response speed higher than that obtained in a conventional twisted nematic mode and that can adopt vertical alignment treatment to dispense with a rubbing process which may cause defects such as electrostatic destruction. Particular attention has been paid to a multi-domain VAN mode (to be referred to as an MVA mode hereinafter) in which the viewing angle can be easily enlarged.
Such a multi-domain structure is generally obtained by electrode-slits or projections for alignment division (for example, see Japanese Patent No. 2565639). The electrode-slits are located at pixel electrodes on an array substrate and the projections are located at a counter electrode on a counter substrate to control gradient of an electric field applied to a pixel area from the pixel and counter electrodes. In this case, the pixel area of the liquid crystal layer is divided upon application of a voltage into, e.g., four domains in which the alignment directions of liquid crystal molecules make angles of 90°. In this manner, an improvement in symmetry of the viewing-angle characteristic and suppression of an inversion phenomenon are realized. A negative optical retardation plate is used to compensate for the visual or observation angle dependence of a retardation produced in a liquid crystal layer in a black display state where liquid crystal molecules are aligned vertical to the electrode substrate, so that the contrast ratio (CR) to a visual angle is made preferable. When the negative optical retardation plate is a biaxial retardation plate having in-plane retardation which compensates for the visual angle dependence of a polarizer, further an excellent visual-angle-contrast characteristic can be realized.
In the MVA mode, however, visual-angle compensation is not sufficient for all gradations but the (minimum) gradation for a black display. In consequence, the luminance (transmittance) characteristic that the panel presents when observed in an inclined direction differs from that the panel presents when observed in the frontal direction of the panel. When the liquid crystal display device is in the MVA mode in which a pixel area is divided into four domains, for example, a visual-angle-luminance characteristic shown in FIGS. 24 to 26 is obtained under liquid-crystal application voltages for various gradations, where a visual angle of 0° represents the frontal direction of the panel. FIG. 24 shows a visual-angle-luminance characteristic measured by shifting the visual angle right and left from the frontal direction of the panel, FIG. 25 shows a visual-angle-luminance characteristic measured by shifting the visual angle diagonally from the frontal direction of the panel, and FIG. 26 shows a visual-angle-luminance characteristic measured by shifting the visual angle upward and downward from the frontal direction of the panel. In this case, the liquid-crystal application voltage is selected from the range of 0 to 4.7 V and applied to a liquid crystal layer LQ. In FIGS. 24 to 26, the abscissa indicates the visual angle where 0° corresponds to the frontal direction of the panel. The ordinate indicates the luminance of the panel in terms of transmittance. According to the visual-angle-luminance characteristic, a difference in luminance between intermediate gradations decreases when the visual angle is determined to observe the panel in a direction inclined from the frontal direction, and the luminance obtained at the (maximum) gradation for a white display is reduced by the visual angle dependence. This raises the problem that a multicolor image entirely looks brownish-white.
Further, another problem is raised by the influences of projections or electrode-slits for obtaining four domains in the MVA mode and Schlieren alignment created on an alignment boundary between these domains. That is, the luminance of the liquid crystal display device is considerably degraded in comparison with the case in which alignment division is not performed. This problem may be solved by reducing the number of divided domains. However, it is difficult to use this countermeasure because of the following reason. When the number of divided domains is four, the liquid crystal display device has a visual-angle-contrast characteristic as shown in FIG. 30. The visual-angle-contrast characteristic is excellent in having contrast ratios (CR) of 10 or more in all directions.
When the number of divided domains is two, the liquid crystal display device has a visual-angle-contrast characteristic as shown in FIG. 31. The visual-angle-contrast characteristic is equivalent to that of the liquid crystal display device in which the number of divided domains is four. However, the liquid crystal display device has a visual-angle-luminance characteristic as shown in FIGS. 27 to 29. According to these drawings, it is understood that luminance inversion occurs at intermediate gradations. The anisotropies of the retardation at intermediate gradations are compensated for when observed in a direction inclined upward or downward since the liquid crystal molecules have antiparallel alignment of the two domains. In contrast, the anisotropies uniformly affect each other when observed in a direction inclined left or right. More specifically, the luminance inversion occurs because a degree of change in the retardation according to the application voltage varies with the visual angle to the frontal direction of the panel. Therefore, when the number of divided domains is two, the luminance of the liquid crystal display device is improved, but the visual-angle-luminance characteristic of the liquid crystal display device is degraded.